1. Field of the Invention
The present invention relates to a method and apparatus used in transfer molding to provide a flowable resin to a substrate having one or more semiconductor devices thereon for the packaging thereof. More specifically, the present invention relates to a method and apparatus used in transfer molding to prevent voids and air pockets in a flowable resin provided to a substrate having one or more semiconductor devices thereon for the packaging thereof.
2. State of the Art
At present, transfer molding is a widely adopted method for plastic encapsulation of semiconductor devices. In transfer molding, the mold generally includes a lower half and an upper half. The lower half of the mold will typically include multiple cavities and a concave portion, called a pot, which communicates with the multiple cavities through runners. In one instance of molding a package, a lead frame at the outer periphery of a semiconductor device is placed on an edge of each of the cavities. The upper half of the mold is placed on the lower half of the mold and includes cavities and a through hole corresponding to the cavities and the pot of the lower half of the mold, respectively. The cavity of either the upper half or lower half, or both, includes a vent, usually at the opposite end of the cavity from the runner, to allow air to push therethrough. An assembly of a semiconductor device and a lead frame connected thereto is arranged in each space defined between corresponding cavities of the lower half and upper half of the mold, where each of the cavities are oriented longitudinally along the horizontal plane. A thermosetting resin is heated in the pot and fed therefrom by a plunger. The resin reaches the cavities through the runners and covers the semiconductor device and a portion of the lead frame located in each of the cavities, pushing air from the runners and cavities through the vent. The resin is typically then heated to cure the same, thus encapsulating the semiconductor device and adjacent portion of the lead frame.
However, as shown in drawing FIG. 1, when the resin or underfill material 1 flows to fill the horizontally oriented cavities 3, the flow is usually not uniform due to various design factors of the semiconductor device 32 and lead frame 33 and gravity acting on the resin 1. As a result, the fronts 1a, 1b of the resin 1 flowing above and below the semiconductor device 32 will often meet above the semiconductor device 32 instead of at the vent, causing the molded package to have undesirable air pockets and/or voids 2, as shown in drawing FIG. 2. These types of defects not only degrade the outer appearance of the molded package, but also produce reliability problems with respect to its resistance to thermal shock and exposure to humidity and other contaminants.
In an effort to prevent such defects in the molded package, U.S. Pat. No. 4,900,485 to Murakami discloses a method and apparatus for transfer molding semiconductor devices including a hydraulic pressure controller and a pressure detector to control the pressure of the resin therein. Additionally, other methods made to prevent defects such as voids by controlling the temperature of the resin are disclosed in U.S. Pat. No. 4,908,178 to Nakagawa et al. and U.S. Pat. No. 5,071,334 to Obara. Although each of the above references disclose a method and/or apparatus for limiting air pockets and/or voids in a molded package, each reference discloses a transfer mold that is longitudinally oriented to be horizontal. Thus, as previously discussed, there remains the problem of non-uniform flow fronts in the transfer mold, resulting in air pockets and/or voids.
Along with the previously discussed problems in transfer molding, methods in flip-chip packaging are known to present similar problems of voids and/or air pockets in underfill material 1 in a gap between a bumped semiconductor die and a substrate. As shown in drawing FIG. 3, such methods include a one-sided or two-sided dispense process, where an underfill material, such as resin 1, is dispensed along one or two adjacent sides of the semiconductor die 52. The underfill material 1 then freely flows by capillary action between the semiconductor die 52 and substrate 64, pushing air existing in the gap between the die 52 and the substrate 64 from opposing sides of the semiconductor die 52 as the underfill material 1 fills the gap, thereby minimizing potential voids. However, as shown in drawing FIG. 3, the underfill material 1 will often leave air pockets or voids 2 adjacent the conductive structures 56 of the flip-chip semiconductor die 52. Further, it is desirable to improve the time it takes to fill the gap with the underfill material.
U.S. Pat. No. 5,766,982 to Akram et al. addresses improving the time for underfilling a flip-chip package by elevating the package on an inclined plane from a horizontal plane. In this method, the underfill material is dispensed either through an aperture in the substrate or at one or two elevated sides of the gap between the die and substrate. When dispensing the underfill material by these methods, the underfill material filling the gap flows down the inclined plane, thereby utilizing gravity to decrease the time necessary for underfilling. Although this method improves the time for underfilling, there remains the potential for voids and air pockets to form due to non-uniform flow and, in particular, voids forming adjacent the bumps of the flip-chip package.
Therefore, it would be advantageous to obtain substantially uniform flow of a packaging or underfill resin to reduce or eliminate the occurrence of voids in the resin, as well as to provide molds and systems that facilitate uniform resin flow during packaging or underfill operations.
The present invention relates to a method and apparatus for limiting voids in a flowable packaging material provided to a substrate, such as a semiconductor die or a wafer or other large-scale substrate including a plurality of semiconductor devices fabricated thereon. The present invention is directed to a method and apparatus for providing the flowable material to the substrate in a substantially vertical direction with respect to a horizontal plane. The method of the present invention includes orienting the substrate substantially vertically.
In one embodiment, the present invention includes a transfer mold having at least one cavity with a gate at a lower portion of the cavity and a vent at an upper portion of the cavity, in which the cavity may be oriented longitudinally perpendicular to the horizontal plane and configured to vertically orient the substrate. According to this embodiment of the present invention, the flowable material fills the cavity from the gate at the bottom thereof to the vent at the top thereof to encapsulate the substrate and/or provide a layer of the flowable material over at least a portion of the substrate. By this arrangement, voids and air pockets are substantially prevented from forming in the flowable material since the flowable packaging material fills the cavity in the vertical direction and due to the force of gravity acting on the flowable material.
In another embodiment, the present invention includes a semiconductor device attached to a substrate having bumps therebetween, in which the bumps provide a gap between the semiconductor device and the substrate. According to this embodiment of the present invention, the semiconductor device and substrate are oriented substantially vertically so that the gap may be filled with flowable underfill material in a vertical direction. This embodiment may include a barrier positioned about the periphery of the semiconductor device for containing the flowable material in the gap between the semiconductor device and the substrate. The barrier includes an opening where the flowable material is introduced into the gap. The flowable material fills the gap from the bottom thereof and is drawn upwardly therethrough by way of capillary action or under positive or negative pressure.